1. Field of the Invention
This invention relates to a method and apparatus for composting. More particularly, this invention relates to continuous and cost-effective composting of large quantities of waste material while maintaining maximum control over the reaction parameters by using closed loop air circulation.
2. The Prior Art
General methods and apparatuses for composting have existed many years. Composting may even be accomplished without any particular apparatus at all. For instance, windrows can be used. Windrows is the composting of a material by laying it out on a field and periodically turning it over with a tractor However, windrows suffer from a number of deficiencies. First, windrows are highly susceptible to adverse weather conditions. Furthermore, the biological and chemical makeup of the material to be composted cannot be assayed and used to adjust the composting parameters. Mixing of the windrowed material may only be accomplished by manually overturning the windrows. Manual overturning often leaves partially composted material in a non-homogenous state. This non-homogeneity leads to non-uniform temperature distribution as well as anaerobic pockets in the material. These pockets create the obnoxious odors associated with open-air composting methods. Therefore, a need exists for an improved composting method which overcomes the deficiencies of windrowing.
Converting sewage sludge into usable humus fertilizer is an environmentally sound goal. This conversion is accomplished via aerobic stabilization and rotting, i.e., composting. In actual use, static methods of composting (such as windrowing) remain unsatifactory on both the technical and economic level. The dissatisfaction has lead to efforts to develop machine composting.
Since the 1970s, composting has become an important method for stabilizing and processing municipal sewage. See EPA, Summary Report on In-Vessel Composting of Municipal Wastewater Sludge, Risk Reduction Engineering Laboratory, Center for Environmental Research Information, September 1989. The technology has developed extremely rapidly, from less than 10 facilities in 1975 to nearly 200 under design or in operation in 1989. Because of odor, labor, and materials-handling problems, designers are producing composting systems built to contain the materials within a vessel. These systems extensively use conveyors and other materials-handling equipment. Although evolution of "in-vessel" systems is very rapid, municipalities continue to face serious problems in dealing with odors, removing moisture, handling the materials in the system and marketing the product.
In-vessel composting is an integrally related system which includes:
Materials (sludge cake, amendment, and recycle) PA1 Materials handling (including storage, mixing and conveyance) PA1 Reactor system PA1 Aeration system PA1 Odor control system PA1 Exterior curing/storage facilities PA1 Marketing
A general composting process begins with the mixing of sludge cake, amendment, if any, (e.g., sawdust) and recycling it in an aerated reactor. Air is diffused into the reactor for temperature control, moisture removal and biological metabolism. Air from within the reactor is then exhausted to an odor treatment system before being dispersed into the atmosphere. After a desired detention time within the vessel, the material is removed from the reactor for further curing/storage.
Composting occurs in multiple stages. The first stage is a high-rate phase. This phase is characterized by high oxygen uptake rates, high temperatures, rapid degradation of biodegradable volatile solids and high odor production. The second stage is a curing phase. This phase is characterized by lower temperatures, reduced oxygen uptake rates and a lower, but significant, potential for odor production.
In machine composting methods, the user can control mixing, ventilation, oxygen supply, moisture content and temperature to more reliably, rapidly and economically transit the two phases and, therefore, perform composting. As noted above with respect to windrows, the major problem with composting is the formation of anaerobic zones as a result of insufficient mixing. Despite numerous efforts, none of the existing machine solutions provides a truly simple, economical, elegant solution to the general problems associated with composting.
Other in-vessel composting apparatuses solve, at least to some degree, some of the problems of windrowing. For instance, the device of U.S. Pat. No. 3,533,775 (the '775 patent) to Brown provides a process for aerobically preparing fertilizer from a mixture of waste containing paper and sewage sludge. Salvageable materials in the municipal waste are "manually, pneumatically, mechanically, or electromagnetically removed." The remainder is comminuted and any paper and film plastic is removed and burned. The heat from burning is used to evaporate water from the sewage sludge or for drying the compost. The remaining waste is then deposited in a series of 1/2" thick layers in a composting tank. Sewage sludge is added. Oxygen-enriched air is introduced through a false bottom to accelerate aeration. An agitator is used to mix the compost and accelerate decomposition. The compost is finally dried with hot air, ground, and bagged as fertilizer.
However, the device disclosed in the '775 patent does not use a rotating vessel. The lack of rotation of the entire vessel necessitates the use of an external agitator. The likelihood of anaerobic pockets is substantial. Furthermore, it is unlikely that homogenous aeration will occur considering the density of partially composted materials.
U.S. Pat. No. 3,272,740 (the '740 patent) to Gitchel et al. discloses a process for treatment of sewage sludge. First, sewage sludge solids are cured by either settling on a sand bed or by rotary drum filtration and then are dried to obtain a completely dry product. Wet air oxidation reduces the biochemical and chemical oxygen demand of the sewage sludge. Wet air oxidation involves oxidation at 100.degree.-150.degree. C. with gaseous oxygen in the presence of water. In wet air oxidation, some of the water is kept in a liquid state by performing the oxidation under pressure. The '740 device achieves a continuous partial wet air oxidation. To produce a "desirable, innocuous organic sludge," the oxygen supplied during wet air oxidation is limited to about 5 to 45 percent of the chemical oxygen demand of the sludge. A continuous flow of a source of known chemical oxygen demand is varied for purposes of equilibriating the oxygen consumption in the reaction chamber. The '740 patent also discloses the variation of gaseous oxygen flow rate while maintaining a constant sludge flow rate. Heat exchange is used for temperature regulation.
Unlike the device disclosed in the '775 patent, the device disclosed in the '740 patent is capable or air-tight and even pressurized operation. In addition, limitation of oxygen supply is discussed and the device is capable of continuous operation. However, no means for removing liquids or gases from the vessel is disclosed. Most significantly, wet air oxidation is an entirely different process from composting. Wet air oxidation is a chemical process. Composting is a biological process. The temperature ranges needed for wet air oxidation are unsuitable for composting. Therefore, there remains a need for a biological composting apparatus which is capable of continuous pressurized operation and into and from which reactants such as air and water can be added or removed as appropriate.
U.S. Pat. No. 2,929,688 (the '688 patent) to Riker et at. discloses an aerobic composing apparatus. Sewage sludge is first passed through a liming tank and is then dewatered before being passed to a mixer. The mixer combines garbage and sewage sludge and, in addition, heats the mixture. The raw organic material to be composted is then conveyed to the composting tanks and gravity dropped into the upper compartments of the tanks. The tanks are made up of a series of vertically spaced compartments. Each compartment has an exhaust vent to assist in circulation and the exhaust of air and gases. A liquid delivery line is also attached to each compartment for bringing water or other compounds into the compartment. Air lines are positioned across each compartment and compressed air is fed into the compartment through these lines. Each compartment contains a mechanical agitator. Finally, a screw type conveyor is used to move organic material.
The '688 device consists of vertically disposed vessels and, thus, agitation occurs only via mechanical means within the vessel. The device is not air-tight. The energy costs of manual agitation make systems like the '688 system inefficient.
U.S. Pat. No. 4,072,494 (the '494 patent) to Gujer discloses another device for composting of high water content sludge. The device involves mixing the sludge with oxygen-containing gas under pressure in a gas impermeable device. Excess heat is removed in the device disclosed in the '494 patent by simply cooling the device's walls. Cooling or heating air is also disclosed as a way to accomplish heat exchange. The device itself is shaped as a gas-tight drum having a horizontal axis of rotation. The rotation is accomplished using rollers and the drum rotates at about 10 to 15 RPM. Scoop-like protuberances are present on the interior walls of the drum and are used to scoop up the substantially liquid sludge and thereby aerate it.
The device disclosed in the '494 patent, however, would be unsuitable for composting low water content biomass. The device disclosed in the '494 patent lacks means for controlling liquid flow and also lacks means for moving partially composted material within the composter. No staging process is contemplated. This is understandable because the high water content sludge contemplated for use in the device disclosed in the '494 patent presents a substantially higher viscosity than normal composting materials. Therefore, gravity flow would be sufficient to move the material along the drum. In addition, maintaining the high rotational speed of the composter is likely quite expensive in terms of energy needed, thereby decreasing the efficiency of the system. Furthermore, the use of scoops is only useful for high water content sludge. For substantially solid biomass, these scoops would either fail to perform their intended function or would mechanically fail from the enormous stresses placed upon them by moving the biomass. The device disclosed in the '494 patent is a wet composting device, operable on materials having from 5 to 8% (but no more than 25%) solids content. Biological degradation, however, requires a maximum of 22% water content for viability of active bacteria.
Therefore, it is highly desirable to provide a composter capable of continuous operation upon a substantially solid biomass having a high solids content and, therefore, low viscosity. It is desirable that such a device allow for control over liquid content within the vessel, as well as having the positive capability to move material from one part of the vessel to another. It is also desirable to have a device which is capable of operating with a minimum of energy overhead.